Question: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $t \neq 0$. $p = \dfrac{3t^2 - 24t}{t^2 - 11t + 24} \div \dfrac{-6t - 24}{t - 3} $
Answer: Dividing by an expression is the same as multiplying by its inverse. $p = \dfrac{3t^2 - 24t}{t^2 - 11t + 24} \times \dfrac{t - 3}{-6t - 24} $ First factor the quadratic. $p = \dfrac{3t^2 - 24t}{(t - 3)(t - 8)} \times \dfrac{t - 3}{-6t - 24} $ Then factor out any other terms. $p = \dfrac{3t(t - 8)}{(t - 3)(t - 8)} \times \dfrac{t - 3}{-6(t + 4)} $ Then multiply the two numerators and multiply the two denominators. $p = \dfrac{ 3t(t - 8) \times (t - 3) } { (t - 3)(t - 8) \times -6(t + 4) } $ $p = \dfrac{ 3t(t - 8)(t - 3)}{ -6(t - 3)(t - 8)(t + 4)} $ Notice that $(t - 8)$ and $(t - 3)$ appear in both the numerator and denominator so we can cancel them. $p = \dfrac{ 3t(t - 8)\cancel{(t - 3)}}{ -6\cancel{(t - 3)}(t - 8)(t + 4)} $ We are dividing by $t - 3$ , so $t - 3 \neq 0$ Therefore, $t \neq 3$ $p = \dfrac{ 3t\cancel{(t - 8)}\cancel{(t - 3)}}{ -6\cancel{(t - 3)}\cancel{(t - 8)}(t + 4)} $ We are dividing by $t - 8$ , so $t - 8 \neq 0$ Therefore, $t \neq 8$ $p = \dfrac{3t}{-6(t + 4)} $ $p = \dfrac{-t}{2(t + 4)} ; \space t \neq 3 ; \space t \neq 8 $